After playing a bit with my main kit, I’ve decided to mount my second Core on the free Di board.
I’ve shorted the pads for R6, R8 and R10, soldered the AMS1117 and soldered the Core along with small header pins on serial pins.
When powered, nothing shown on the serial … 
Is the firmware pre-uploaded on every Cores ?
Why it doesn’t boot ?
I’ve got it !
Probable a bad connection on the cable of my USB-TTL …
Good!